Molecular Biology of the Gene

Gene Expression: From DNA to Protein

Gene expression is the process by which information encoded in a gene is used to direct the synthesis of a functional gene product, typically a protein. This process involves two main stages: transcription and translation. The final protein determines the trait expressed in an organism.

• DNA is organized into chromosomes, which contain hundreds or thousands of genes.

• The genetic code is written in nucleotide bases: A (adenine), T (thymine), C (cytosine), and G (guanine).

• Gene expression begins when a gene is transcribed into messenger RNA (mRNA).

• The mRNA sequence determines the amino acid sequence of the protein.

• Example: The gene for flower color is transcribed and translated to produce a pigment protein.

Transcription: DNA to mRNA

Transcription is the process by which a segment of DNA is copied into mRNA by the enzyme RNA polymerase. This is the first step in gene expression.

• RNA polymerase unzips the DNA and synthesizes a complementary mRNA strand in the 5' to 3' direction.

• Base pairing rules for transcription:

  • C pairs with G

  • A pairs with U (uracil replaces thymine in RNA)

  • T pairs with A

  • G pairs with C

• Transcription occurs only when a gene is being expressed.

Stages of Transcription

• Initiation: RNA polymerase binds to a region called the promoter on the DNA, which signals where transcription should start and which strand to transcribe.

• Elongation: RNA polymerase moves along the DNA template strand, adding complementary RNA nucleotides and elongating the mRNA molecule.

• Termination: RNA polymerase reaches a DNA sequence called the terminator, which signals the end of transcription. The mRNA molecule is released.

• Example: The promoter region ensures that only specific genes are transcribed at specific times.

Processing of Pre-mRNA in Eukaryotes

Before mRNA leaves the nucleus, it undergoes several processing steps to become mature mRNA.

• 5' Cap: A modified guanine nucleotide is added to the 5' end. This cap helps the ribosome recognize where translation should begin.

• Poly-A Tail: A sequence of 150-200 adenine nucleotides is added to the 3' end. This tail facilitates mRNA export from the nucleus and protects it from degradation by hydrolytic enzymes.

• Splicing: Non-coding regions called introns are removed, and coding regions called exons are joined together by a complex called the spliceosome.

• Example: Introns may contain mutated or viral DNA sequences that do not code for proteins.

Translation: mRNA to Protein

Translation is the process by which the mRNA sequence is decoded to build a polypeptide (protein) at the ribosome. This occurs in the cytoplasm.

• Ribosomes are the site of protein synthesis and are composed of ribosomal RNA (rRNA) and proteins.

• Ribosomes have a binding site for mRNA and three binding sites for transfer RNA (tRNA): A (amino acid), P (peptide), and E (exit).

• tRNA molecules bring amino acids to the ribosome. Each tRNA has an anticodon that pairs with the mRNA codon.

• The sequence of mRNA codons determines the order of amino acids in the polypeptide.

• Example: The codon AUG codes for methionine and signals the start of translation.

Stages of Translation

• Initiation: The small ribosomal subunit, mRNA, initiator tRNA (with anticodon UAC and amino acid methionine), and initiation factors assemble. The large subunit joins, and translation begins at the P site.

• Elongation: tRNA molecules enter the A site, move to the P site to add their amino acid to the growing polypeptide, and then move to the E site to exit. The polypeptide chain grows longer.

• Termination: When the ribosome encounters a stop codon (UAA, UAG, or UGA), translation ends. The ribosome releases the mRNA and the completed polypeptide.

• Example: Stop codons do not code for amino acids; they signal the end of translation.

Ribosome Structure and Function

Ribosomes are composed of two subunits (large and small) made of rRNA and proteins. They facilitate the pairing of tRNA anticodons with mRNA codons and catalyze peptide bond formation.

• Small subunit: Binds to mRNA.

• Large subunit: Contains the A, P, and E sites for tRNA binding and peptide bond formation.

• Peptide bond formation: Occurs in the large subunit, joining amino acids into a polypeptide chain.

• Example: The ribosome acts like an assembly line, moving tRNAs through the A, P, and E sites.

Gene Expression and Protein Targeting

Once a protein is synthesized, it may remain in the cell or be secreted, depending on signal sequences in the polypeptide. Proteins may undergo further processing, such as addition of sugars, phosphates, or lipids, and are transported by vesicles to their final destination.

• Signal sequences: The first few amino acids in a polypeptide act as signals for cellular localization.

• Exocytosis: Proteins destined for secretion are transported in vesicles and released from the cell.

• Example: Hormones are synthesized in the cell and secreted via exocytosis.

Summary Table: Steps in Gene Expression

Key Terms and Definitions

• Gene Expression: The process by which a gene's information is used to synthesize a functional product.

• Transcription: Synthesis of RNA from a DNA template.

• Translation: Synthesis of a polypeptide from an mRNA template.

• mRNA: Messenger RNA; carries genetic information from DNA to the ribosome.

• tRNA: Transfer RNA; brings amino acids to the ribosome during translation.

• rRNA: Ribosomal RNA; structural and catalytic component of ribosomes.

• Codon: Three-nucleotide sequence in mRNA that specifies an amino acid.

• Anticodon: Three-nucleotide sequence in tRNA complementary to the mRNA codon.

• Introns: Non-coding regions of a gene removed during mRNA processing.

• Exons: Coding regions of a gene joined together during mRNA processing.

• Promoter: DNA sequence where RNA polymerase binds to start transcription.

• Terminator: DNA sequence signaling the end of transcription.

• Initiation Factors: Proteins that help assemble the translation machinery.

• Polypeptide: Chain of amino acids; precursor to a functional protein.

Important Equations and Concepts

• Central Dogma of Molecular Biology:

• Base Pairing in Transcription:

• Start Codon:

  • (codes for methionine)

• Stop Codons:

  • , ,

Additional info: The notes expand on the transcript by providing definitions, examples, and a summary table for clarity. Figures referenced (12.12, 12.14, 12.16, 12.17, 12.19, 12.20) are from a standard biology textbook and illustrate the described processes.